The present invention relates to a method of manufacturing micromechanical surface structures by means of a vapor-phase etching medium, avoiding aluminum contact corrosion.
In silicon surface micromechanics, a layer structure is used, consisting of a sacrificial layer, usually SiO2, on a substrate surface, usually a silicon substrate, with a layer of active silicon, usually polysilicon or single-crystal silicon (SOI), on top, in which structures, to be exposed later, are produced. Additional layers, e.g., buried polysilicon printed conductors, may also be present, but they do not play any role in the purely mechanical function of the components. In general, metal contact surfaces are provided on the active silicon layer for electric contacting of the components. Various aluminum alloys (AlSi, AlSiCu, etc.) containing aluminum as the predominant element have become established as contact materials in semiconductor technology, the repertoire of which has been used preferentially in surface micromechanics.
It is known that to produce unsupported micromechanical surface structures, they are first etched into the top layer of active silicon until the underlying sacrificial layer, usually SiO2, is reached (M. Bibel, Physikalische Blxc3xa4tter [Physical Leters], 1996, 52, pp. 1010-1012). To expose the structures created in this way, the sacrificial layer is dissolved, for example, by an isotropic wet etching method (PCT International Publication No. WO 92-03740), where gaseous hydrogen fluoride over an azeotropic hydrofluoric acid-water mixture can be used (M. Offenberg, B. Elsner and F. Lxc3xa4rmer, Proc. 186th Electrochem. Soc. Meeting, Sensor General Session, Miami Beach, Fla., October 1994).
HF vapor is known to convert SiO2 to volatile silicon fluorides, thereby dissolving it away under the structures:
xe2x80x83SiO2+2H2O+4HFxe2x86x92SiF4+4H2O
The presence of water is necessary for this reaction to take place. It can be seen from this reaction equation that more water is formed than used. The important advantage of using gaseous hydrogen fluoride in comparison with aqueous hydrofluoric acid solutions is that with an optimum choice of measurement parameters there is no irreversible sticking of the exposed silicon structures to one another or to the substrate due to the surface tension of drying droplets of fluid in the subsequent drying of the substrate.
An important disadvantage of this vapor-phase etching process is that the gaseous hydrogen fluoride attacks not only SiO2 but also the aluminum contacts that have been applied to the electronic components. In vapor-phase etching, the aluminum hydroxide fluorides produced cannot be removed but instead they remain as a rather thick insulating layer on the contact surface, which makes subsequent wire bonding of the aluminum contacts impossible. Washing off this layer would in turn result in sticking of the micromechanical surface structures already exposed and therefore is also impossible. Another problem with the resulting aluminum hydroxide fluoride layers is that they are hygroscopic, and water absorbed penetrates to the interlayer of metallic aluminum with the aluminum hydroxide fluoride layer, thus leading to progressive corrosion both during and after the etching of the sacrificial layer is completed.
The aluminum contacts may be protected during the etching of the sacrificial layer by means of additional layers such as lacquers that are impermeable to hydrofluoric acid, but this represents an additional step that is complicated and technically very difficult during the process of manufacturing the micromechanical surface structures because hydrofluoric acid diffuses very rapidly through protective polymer layers and thus can reach the metal surface. In addition, the contact protection would also have to be removed again after etching the sacrificial layer, i.e., if already exposed sensitive structures are present on the wafer surface, which leads to additional problems, in particular with regard to yield and reproducibility.
By establishing a temperature difference between the substrate and the vapor phase of the etching medium on the basis of the partial pressure composition of the vapor-phase etching medium, it is possible to control the chemical reactions during etching in an especially advantageous manner. A temperature difference between the substrate and the vapor phase permits selective etching of SiO2 on the basis of the reaction below without attacking the exposed aluminum contacts on the substrate. The first reaction which can take place on the aluminum surface is then as follows:
Al2O3.3H2O+2HFxe2x86x922Al(OH)2F+2H2O⇄2Al(OH)3+2HF
In the course of this reaction, the aluminum oxide hydrate breaks through at the surface. The hydroxide or hydroxide fluoride layer is hygroscopic. First, metallic aluminum is converted by the action of water under the influence of hydrofluoric acid to the oxide hydrate which can be fluorinated further by the following equation:
Al+3H2O+HFxe2x86x92Al(OH)2F+3/2H2+H2O⇄Al(OH)3+3/2H2+HF
Aluminum hydroxide and aluminum hydroxide fluoride are in a reversible chemical equilibrium. The conversion of aluminum to the corrosion product takes place essentially with the uptake of water, in contrast with SiO2 etching, where water is formed. Both reactions, i.e., etching aluminum and SiO2, have in common the fact that they can take place only in the presence of water. Establishing a temperature difference between the substrate and the vapor phase of the etching medium permits, for example, rapid vaporization of the water formed on the substrate surface. Owing to the temperature difference, which is due to the partial pressure composition of the vapor-phase etching medium, water cannot condense on the substrate, and the parts of the surface which do not produce any water in the reaction with hydrofluoric acid, for example, remain dry and cannot be attacked.
In an advantageous embodiment of the method according to the present invention, the etching is performed at a temperature difference of 10-30 K, preferably 20 K, between the silicon substrate and the vapor-phase etching medium. The temperature of the vapor phase opposite the substrate is lower than the temperature of the substrate, so there is no condensation on the substrate surface. Consequently, the substrate surface is exposed to the vapor phase, but due to the higher substrate temperature, there cannot be any condensation on the substrate, and the parts of the substrate that cannot produce any water themselves in the reaction with hydrofluoric acid remain dry and cannot be attacked. This is true especially of the aluminum of the electric contacts which also does not release any water in the reaction with aqueous hydrofluoric acid, and therefore it does not react due to the absence of moisture. However, in the reaction with HFxe2x80x94H2O, the sacrificial SiO2 layer reacts by forming water, some of which is bound as hydroxide in the form of Si(OH)4, as a precursor to additional reactions with HF to form volatile silicon tetrafluoride. The portion of the reaction water that is not bound as a hydroxide remains on the SiO2 surface for a relatively short period of time and is rapidly evaporated because of the higher wafer temperature in comparison with the vapor phase.
In any case, even the transient presence of this reaction water accelerates the subsequent reaction of SiO2 (or then Si(OH)4) with HF, which supplies even more water for the reaction, until an equilibrium moisture content of the SiO2 and Si(OH)4 surfaces prevails. This acceleration of the SiO2 etching process from initially very minor removal of material, which is initiated only by the water from the vapor phase, up to high etching rates results in achieving a high, quasi-steady-state silicon oxide etching rate after a start-up phase of approximately 5 to 7 minutes after the start of the process.
In another preferred embodiment, the temperature of the silicon substrate is at least 333 K, preferably 343-353 K, in particular 353 K ad at most 373 K. Consequently, there is no avalanche of dissolution reaction due to uptake of water from the gas phase on the dry aluminum surfaces that can form no reaction water and thus there is also no significant corrosion. At this temperature, additional protective mechanisms for the aluminum come advantageously into effect, resulting the fact that no fluorides remain on the surface. Thus, any delayed corrosion is effectively prevented even long after the actual etching of the sacrificial layer. The reliability of the components produced in this way is thus greatly improved.
Aluminum oxide hydrate, aluminum hydroxide or aluminum hydroxide fluoride present on the surface is dehydrated, i.e., water or the water of crystallization is removed in the form of hydroxides, or also HF; fluorine in the form of its fluorides is thus completely removed from the surface layer.
Al2O3.3H2Oxe2x86x92Al2O3+3H2O
2Al(OH)3xe2x86x92Al2O3+3H2O
Al(OH)2Fxe2x86x92AlO(OH)+HF
Structural compaction of the layer passivating the aluminum surface occurs in this dehydration, i.e., its pore density and permeability decrease, while its imperviousness and passivation with respect to water and HF increase. The chemical resistance of the layer passivating the aluminum surface, i.e., its protective effect, is thus increased in particular by the formation of compounds such as aluminates which are more chemically stable and inert. This effect is especially operative above a temperature range of 343-353 K. Up to a substrate temperature of 373 K, strong SiO2 etching can still be performed, without any aluminum contact corrosion occurring. At temperatures in the preferred temperature range of 343-353 K, especially at 353 K, there is no longer any fluorination of the aluminum surface, i.e., the aluminum surfaces do not contain any fluorine atoms or ions after etching.
In another preferred embodiment, the vapor phase is adjusted so that another gas which is essentially chemically inert under the selected reaction conditions is used and is introduced into the etching apparatus. The establishment of a xe2x80x9cquasi-azeotropicxe2x80x9d mixture is thus regulated easily through the moisture in this gas. The chemically inert gas for dilution may also contain oxygen, so it is even possible to use air.
In another embodiment of the present invention, the partial pressure composition of the etching medium can be adjusted as a function of a temperature of the etching medium and/or as a function of a composition of components of the vapor phase.